PCT/EP 00/08703 describes a method and apparatus for controlling the level of laser beam energy of a laser beam scanning across a target using at least one laser scanhead, said laser scanhead comprising motor driven deflection means for scanning the laser beam across the target and polarization control means, wherein the polarization control means controls the level of laser beam energy of the laser beam scanning across the target in accordance with the movement of the deflection. In particular, the polarization control means includes one or two Brewster plates or Brewster windows which are rotatable around an axis which is parallel to the direction of the laser beam. The one or two Brewster plates can be rotated at an angle between 0xc2x0 and 90xc2x0 to control laser beam transmission between 0% and 100%. Because a laser beam in general includes only one type of polarization, a Brewster plate, set at the correct Brewster angle, can be used to transmit between 0% and 100% of said polarization. The rotation of the Brewster plate is synchronized to the rotation of the deflection means, deflecting the laser beam for targeting a scanned surface.
Reference is made to PCT/EP 00/08703 in its entirety.
FIG. 1 shows a diagram of the laser beam targeting apparatus according to PCT/EP 00/08703. The laser beam targeting apparatus is integrated into a scanning head 32 of a laser scanner into which a linearly polarized laser beam is passed through an aperture 34. The scanning head 32 comprises a polarization control device 36 which can be either integral or mounted in the beam axis line preceding the scanning head, a beam expansion optic 38, a Y axis mirror 40, a X axis mirror 42, two galvanometric motors 44 and 46 for rotating the two mirrors 40 and 42, respectively, and an f-theta focusing lens 48. The polarization control device 36 of the embodiment of FIG. 1 comprises a first Brewster plate 50 and a second Brewster plate 52. In an alternative embodiment, this technique could also be used with either pre- or post-objective scanning.
In a further alternative embodiment, a polarization control device comprising a single Brewster plate could be used to achieve basically the same effect, as described above.
In the shown embodiment a carbon dioxide (CO2) laser is used for creating a laser beam linearly polarized in a single direction. However, the expert will understand that any other suitable type of laser source may be used. The laser beam enters the scanning head 32 through the aperture 34 and is passed through the polarization control device 36 in which two opposing ZnSe-Brewster plates 50, 52 are set at the relevant Brewster angles with regard to the laser beam wavelength. The Brewster plates 50, 52 can be rotated through 90xc2x0 to attenuate the laser beam thereby, allowing 0% to 100% of the laser beam energy to be transmitted through said Brewster plates 50, 52 when they are rotated around the laser beam axis from 0xc2x0 to 90xc2x0. For other types of lasers other material might be required for the Brewster plates. Alternative coatings may be used on the face of the Brewster plates which may vary the maximum and minimum levels of transmitivity, the exit polarization, and the required rotation of the Brewster plates. In practice, the maximum transmitivity of a Brewster window of the type described above is xe2x80x9conlyxe2x80x9d 99.98%. However, for the purpose of the present description a transmitivity of 100% may be assumed. Therefore, if in the present text a transmitivity of 100% is indicated, it is referred to the maximum transmitivity of the respective Brewster window, which in the embodiment considered is 99.98%.
The part of the laser beam energy transmitted through the Brewster plates 52, 50 in this embodiment passes through a beam expansion optic 38 which expands the laser beam diameter and is then deflected off the surface of the Y axis galvanometric motor driven mirror 40 to be then deflected off the surface of the X axis galvanometric motor driven mirror 42 and through the f-theta focusing lens 48 which acts to focus the laser beam to a fine point on a target plane 54. The intensity of the laser beam energy scanned across the target plane 54 is held under strict control by controlling the rotation of the Brewster plates 52, 50 as a function of the rotation, position, angular speed of the galvanometric motor driven mirrors 40, 42.
It is important that the travelling time to and from maximum required velocity of the combined XY beam position at the target is matched to the transmission curve of the opening and/or closing of the Brewster rotation. In practice, every travelling time of the beam crossing the target plane in the X or Y direction, and importantly the combined XY directions should be defined. It is assumed that this defined travelling time will be determined by the capability in speed of the polarization control device 36 to open and close the Brewster plates. Therefore, if as an example it takes 1 ms for the Brewster plates to open, within an acceptable tolerance, from 0% to 100% and, equally 1 ms to close, then the scanning head 32 and in particular the combined scanning mirrors 40, 42 should reach the maximum speed in 1 ms. Because coated or enhanced Brewster plates have a transmitivity of 0% when set to the appropriate angle it is not necessary to turn off the laser beam between independent processing or marking steps of the target material.
The PCT application describes the actions of the Brewster windows set at a specific Brewster angle to control laser power by reflecting or transmitting a single directionally polarised carbon dioxide generated laser beam energy. The same method can be applied to any single directionally polarised energy using the correct material for the Brewster windows and the correct Brewster angle specific to the wavelength of said energy.
FIG. 2 details how a laser beam polarised in either the P-pol (parallel) or S-pol (senkrecht or perpendicular) directions can be reflected or transmitted using a single Brewster window 200.
For illustration purpose both P-polarization and S-polarization are shown in the drawings. However, an expert will understand that in practice a CO2 laser beam can comprise basically only one type of polarization. With reference to FIG. 2 if said input polarization is P-pol then the energy will be reflected whilst if said input polarization is S-pol then the energy will be transmitted.
Disadvantageously, said transmitted beam energy will be displaced by a factor calculated by the Brewster angle giving an angle of incidence and by the thickness of the Brewster window itself.
FIG. 3 depicts the Brewster window 200 rotated through 90xc2x0 where now the P-pol is transmitted and the S-pol is reflected. The beam energy is displaced exactly the same as in FIG. 2 except that it has now been rotated 90xc2x0 about the centerline.
FIG. 4 shows how two Brewster windows 450, 452 aligned together allow for the output beam energy path to be the same as the input beam energy path by the actions of the second Brewster window 452 compensating for the displacement created by the first Brewster window 450. In reality and depending upon the coating on the Brewster windows 450, 452 the P-pol reflected off the first Brewster window 450 will be of a very high percentage leaving only a very small percentage to be reflected off the second Brewster window 452.
FIG. 5 depicts the two Brewster windows 450, 452 rotated in unison in the same direction. As the rotation increases the P-pol reflected off the first Brewster window 450 decreases and the P-pol transmission increases. Correspondingly as the rotation increases the S-pol transmission through the first Brewster window 450 decreases and the S-pol reflection increases.
It is important to note that the laser beam energy polarization exiting the first Brewster window 450 is rotated with the rotation of said first Brewster window 450 dependant upon its coating. Therefore any P-pol that has been transmitted through the first Brewster window 450 will be reflected off the second Brewster window 452 and correspondingly any S-pol that has been transmitted through the first Brewster window 450 will be transmitted through the second Brewster window 452.
Therefore, the second Brewster window 452 will have no significant additional secondary effect on energy control except to re-displace the beam energy back onto the same path as the input beam.
Using the method as depicted in FIG. 5 the two Brewster windows 450, 452 must be rotated in unison through 90xc2x0 in order to control transmission from maximum to minimum and to compensate the offset of the laser beam.
Reference U.S. Pat. No. 4,632,512 describes a method for sequentially attenuating, modulating, and polarizing wherein the laser beam source emits a laser beam which is polarized in one direction. The part of the apparatus used for polarization comprises a fixed pair of symmetrically arranged Brewster elements wherein the axis of symmetry of the polarization part of the arrangement is perpendicular to the axis of the laser beam, and the Brewster elements are declined towards the axis of the laser beam at an angle which corresponds to the Brewster angle.
It is an object of the present invention to further improve the method and apparatus for controlling a laser beam.
To solve this problem the present invention provides a method and apparatus for controlling laser power, using at least two Brewster windows which are aligned along an axis which is parallel to the direction of the laser beam and which are rotatable around said axis, wherein the first Brewster window is rotated in one direction and the second Brewster window is rotated in the opposite direction. In one preferred embodiment of the invention two coated Brewster windows are used, each of which have a transmitivity of almost 100% (in practice 99.98%) to 0% when rotated through 90xc2x0. According to the invention, these two Brewster windows only have to rotate each through +/xe2x88x9245xc2x0 to control transmission of the laser beam from maximum to minimum.
According to another embodiment of the invention, uncoated Brewster windows are used which have a transmitivity of about 100% to 25% when rotated through 90xc2x0. The advantage of uncoated Brewster windows is that they are able to transmit high power laser beams, in the range of about 6 kW, whereas the coated Brewster windows would be damaged by such high power and therefore are limited to transmit laser beams having a lower power. When using uncoated Brewster windows, preferably a plurality of pairs of Brewster windows, such as two or three pairs, are aligned along the laser beam axis. The Brewser windows of each pair are rotated in opposite directions through an angle between 45xc2x0 and 90xc2x0, as will be explained in further detail below. Thus, a method and apparatus for controlling high power laser beams is provided which is reduced in costs and complexity.
In other words, the invention relates to a method and an apparatus to finely control or regulate laser energy reaching a target in direct relationship to the velocity of the focussed beam or spot at that target.
The present invention also provides an apparatus for controlling a level of laser beam energy of a laser beam, comprising at least two Brewster windows which are aligned along an axis which is parallel to the direction of the laser beam, and driving means for rotating said Brewster windows around said axis, wherein said driving means are adapted to rotate a first Brewster window in a first direction and a second Brewster window in a second, opposite direction.